MODELING OF FATIGUE SLIP BANDS AND APPLICATION TO NOTCH FATIGUE PROBLEMS

摘要

In the present paper, equivalent stress ratios (R_(EQ)-ratios), which have been proposed as parameters for correspondence between cyclic stress conditions of notched and unnotched specimens, are reviewed. The R_(EQ)-ratios are formulated based on a concept of plastic adaptation hypothesized for a fatigue slip band from a viewpoint of macro-mechanics. A method for diagramming fatigue strength of metals based on the parameter of the R_(EQ)-ratios is newly proposed. The method diagramming together the fatigue strength of notched and unnotched specimens is applied not only to fatigue problems of usually annealed, normalized and heat-treated materials but also to those of severely heat-treated and surface-treated ones. Fatigue strength diagrams are characterized with two types of fatigue strength: σ_(w1) and σ_(w2). The character of σ_(w2) appears not only in specimens with sharp notches but also in unnotched specimens fatigue-tested at lower R_N-ratios. Criteria on fatigue strength σ_(w1) and σ_(w2) are derived from the fatigue strength diagrams and formulated as empirical equations. Characteristics of fatigue slip bands are reviewed and two types of fatigue mechanisms are proposed related with fatigue strength σ_(w1) and σ_(w2) from a viewpoint of micro-mechanics. Consequently, it is found that the hypothesis of plastic adaptation is a very useful and fundamental idea for modeling a fatigue slip band and also for analyzing fatigue data of practical metals and alloys.